Как работи транзисторът като усилвател на звук

when you sing a song into a mic it converts the changes in the air pressure into

changes in electric current now if you attach this to a speaker then it converts these changes in the

current back to sound but you will probably hear nothing and the reason for that is because this

current produced by the microphone is very tiny causing extremely tiny vibrations and as a result you will hear

nothing but if you could somehow increase the strength of the current over here and

keep the pattern exactly the same and now since the current has increased the speaker will vibrate very nicely and

since the pattern is exactly the same you'll pretty much hear his voice and his song

so in order to make this thing successfully work we need a device in between that increases the strength of

the current but at the same time keeps the pattern exactly the same such devices which do that are called

amplifiers and usually these amplifiers are found inside the speakers themselves now

earlier amplifiers were big and bulky which made our speakers our devices big and bulky so in order to build let's say

a pocket radio or maybe to build headphones we needed to make tiny amplifiers

and it was this trio shockley brattain and bardeen working at bell labs realized the key to building a

very tiny amplifier was using doped semiconductors and after a lot of research and experiments

they finally invented the first semiconductor amplifier in around 1947 1948 so let's find out what they did

their idea was to use an n-type semiconductor with a lot of electrons and sandwich in between

a p-type semiconductor with very few holes like this

or or another thing that you could do is take

a p-type semiconductor with a lot of holes and sandwich in between an n-type

semiconductor with very few electrons they called it the transistor the transistor and we'll see the reason

behind this name in the future videos but since this material has n over here then it has a p

and it has an n type again we call this as the npn transistor and similarly if you look at this one you have p

this is the p then the n-type then again the p-type we call this as the p-n-p

transistor and the key to working of this is that this middle region that is this that's the key actually this middle

region that we have the p-type here and the n-type here it should satisfy two conditions one it has to be very thin

and we'll see why in a minute it has to be very thin and two it has to be very lightly doped

very lightly doped as you can see the doping here is much smaller than what you have over

here same thing over here we'll see now that under such circumstances these things

will act like an amplifier so we can look at either the npn or the pnp let's let's look at npn and see how it can act

as an amplifier so here is our mpn transistor let's begin by attaching a power supply across

its ends so let's put some metallic contacts and let's say we attach the positive of the power supply here

let's say about five volts so plus five volt the negative which is usually the ground

we're going to connect that over here this is the ground and by the way i'm not showing the power

supply if it was a real circuit a practical circuit would be like this you would have a positive of the power

supply here the negative of the power supply right over there and that would be connected over here but i'm just

ignoring this part of the circuit it's there of course but i'm ignoring that so we can focus more on the transistor

action so what do you think is going to happen just pause the video and think about this

well since we have a positive over here we might expect the electrons to get pulled out like this

and we might have a current over here electrons flowing like this but in order for that to happen the electrons must

continuously flow from this region into this region as well right we need electron flow everywhere

but can the electrons from this region flow into this region the answer is no the reason is remember that at every pn

junction there is a depletion region which acts like a barrier for the flow of majority charge carriers the

electrons are the majority over here they really like to flow from here to here due to diffusion but the barrier

prevents them and as a result since these electrons can't flow from here to here due to the barrier these electrons

can't get pulled across and there will be no current in the circuit and regardless of what voltage you put even

if you put a 10 volts or a 15 volts over here you can't you don't expect any current so it'll be nothing

now if you really want a current you know what we could do we could attach another terminal over here

you could attach another terminal and put another circuit over here say we apply a positive to this

a pause again i'm not going to draw the entire circuit i'm just going to draw the positive through this let's say we

put about a positive 0.7 volt over here and we'll see in a while why i'm choosing 0.7 volt what do

you think is going to happen i can pause the video and think about this well now if you look carefully notice

the p is connected to more positive than this m this n is grounded and as a result we are forward biasing this

junction so if you take this this junction is being forward biased

and similarly if you look at this junction notice the n is connected to more positive than p and as a result

this is reverse biased this is reverse biased and now remember that in a when a pn junction is reverse

biased it doesn't allow the flow of majority charge carriers and so these electrons and holes can't flow across

but this junction is forward biased under forward bias the majority charge

carriers can flow across they can diffuse into each other and guess what if you remember for silicon if you hit

0.7 volt that's when the depletion region vanishes and as a result these electrons and holes can now easily

diffuse into each other so let me show that so these electrons will start diffusing into the p and of course these

holes will also diffuse over here but i'm going to neglect the holes because they're very very tiny in number anyways

now the question is what's going to happen to these electrons and now we're reaching the climax of the transistor

action all right notice that these electrons can either be pulled out from here because of the positive or

they can be pulled out from here because of the positive here what's going to happen

well remember that whenever we have a forward bias p-n junction in order for those electrons to get pulled out they

have to undergo recombination we have spoken a lot about this in previous videos so if you need more clarity it

would be great to watch that but anyways in order for the electrons to get pulled out they have to undergo recombination

and the recombination chances over here in the transistor is very very small for two reasons one is because this p region

is very lightly doped so there are small number of holes to begin with and as a result the recombination chances are

small but the second reason is that this is very thin and as a result most of the electrons that get injected over here

will find themselves already at this end and as a result they can now be pulled by this voltage

you can also think of it this way you see when they are reaching this this junction because it is reverse

biased remember a reverse bias doesn't allow majority charge carriers to flow through but they accelerate minority

charge carriers right and these electrons in the p region are the minority charge carriers so because of

the electric field they get accelerated and they get collected over here and then this they can you know flow through

the terminal over here so what's happening over here is that most of the electrons which get injected

will just come out from here and only a fraction of them will get recombined and as a result they get pulled out from

here so if you were to put some numbers if you were to put some numbers we could say about 100 electrons are being

injected per second 100 electrons are injected per second and maybe due to recombination only one

electron is being pulled out from this terminal so one electron per second and as a result about 99 electrons get

pulled out from here 99 electrons get pulled out from here which means the current in this wire is

about 99 times the current in this wire and you may be like what's the big deal well the big deal is let's think of it

this way you see in order to pull one electron from here when we try to pull one electron from here about 99

electrons gets pulled out from here right that's the way we can think about this well now imagine if we increase

this voltage and try to pull more electrons from here let's say i tried to pull two electrons from here what will

happen well we have the statistics here we've seen that almost out of hundred one gets

pulled out so in order to get so to remove two about 200 will get injected they have to get injected right you're

increasing the forward bias voltage more will get injected about 200 will get injected two comes out from here that

means 199 198 will get collected over here that means notice when you double this this also has doubled

and if this were to triple this would be triple if this were to half this would be half and so on in other words

if the current in this wire fluctuates the current in this wire would fluctuate in exactly the same manner

however the current over here will always be 99 times more than the current over here

in other words this is this the current in this wire is the amplified version of the current in this wire and that's

exactly what we needed and so if you want to use this to amplify your sound then you can connect your microphone

wire over here now the microphone will be the one that will provide the voltage over here needed to forward bias this

junction and the voltage provided by the microphone will depend upon the sound if your sound is very loud this voltage

will be high if the sound is very low like you're whispering the voltage will be very low over here of course the

engineers will make sure that the voltage over here will never go below 0.7 volt and all of that stuff but don't

worry this voltage will fluctuate pretty much depending upon your sound and as a result the current in this wire will

also fluctuate depending on your sound now guess what the current in this wire will fluctuate in this exactly the same

manner as the current fluctuates over here that's what we saw but it is 99 times more so it's amplifier so if we

feed this now to a speaker then in the speaker the sound generator will be much louder than the sound that

you're producing over here but the pattern of that sound will be exactly the same because the fluctuations are

exactly the same which mean that sound will be exactly like your voice but it'll be much louder

and that's how a transistor can be used as an amplifier